The present invention is concerned with a novel process for the manufacture of an optically active lactone of the formula ##STR3## wherein R is benzyl.
This optically active lactone of formula I is a known, valuable intermediate in the synthesis of (+)-biotin, derivatives thereof and compounds related thereto.
Under the designation "(3aS,6aR)" used in connection with formula I there is to be understood in the scope of the present invention that antipode which is dextrorotatory in benzene or chloroform. This antipode is denoted hereinafter as the (+)-lactone.
A process for the manufacture of the (+)-lactone of formula I is already known from German patent specification No. 2 058 248 (corresponding to U.S. Pat. No. 3,700,659). In this process, racemic hemiesters of the formula ##STR4## wherein R is benzyl and R.sup.1 is cholesteryl or cyclohexyl, are separated into their optical antipodes and a desired antipode (i.e. the (+)antipode) is converted into the (+)-lactone of formula I. However, this process has the disadvantage that an undesired antipode, which results from the racemate resolution, must be recyclized before it can be converted to compound I.
There accordingly exists a need for a process wherein this undesired antipode can be converted into the desired (+)-lactone of formula I without recyclization. This need has been satisfied by the present invention. It has surprisingly been found that this undesired antipode, after conversion into the corresponding acid chloride, can be reduced selectively with a dialkylaluminum hydride or a complex borohydride to give the (+)-lactone of formula I.